Light olefins, such as ethylene, propylene, butylenes and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, C2-C4 light olefins are produced by cracking petroleum refinery streams, such as C3+ paraffinic feeds. In view of limited supply of competitive petroleum feeds, production of low cost light olefins from petroleum feeds is subject to waning supplies. Efforts to develop light olefin production technologies based on alternative feeds have therefore increased.
An important type of alternative feed for the production of light olefins is oxygenates, such as C1-C4 alkanols, especially methanol and ethanol; C2-C4 dialkyl ethers, especially dimethyl ether (DME), methyl ethyl ether and diethyl ether; dimethyl carbonate and methyl formate, and mixtures thereof. Many of these oxygenates may be produced from alternative sources by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastic, municipal waste, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum sources for light olefin production.
The preferred process for converting an oxygenate feedstock, such as methanol, into one or more olefin(s), primarily ethylene and/or propylene, involves contacting the feedstock with a crystalline molecular sieve catalyst composition. Crystalline molecular sieves have a 3-dimensional, four-connected framework structure of corner-sharing [TO4] tetrahedra, where T is any tetrahedrally coordinated cation. Among the known forms of molecular sieve are aluminosilicates, which contain a three-dimensional microporous crystal framework structure of [SiO4] and [AlO4] corner sharing tetrahedral units silicoaluminophosphates (SAPOs), in which the framework structure is composed of [SiO4], [AlO4] and [PO4] corner sharing tetrahedral units.
Molecular sieves have been classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework-type zeolite and zeolite-type molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the Atlas of Zeolite Framework Types, 5th edition, Elsevier, London, England (2001), which is herein fully incorporated by reference.
Among the molecular sieves that have been investigated for use as oxygenate conversion catalysts, materials having the framework type of the zeolitic mineral chabazite (CHA) have shown particular promise. For example, SAPO-34 is a crystalline silicoaluminophosphate molecular sieve of the CHA framework type and has been found to exhibit relatively high product selectivity to ethylene and propylene, and low product selectivity to paraffins and olefins with four or more carbon atoms.
The preparation and characterization of SAPO-34 have been reported in several publications, including U.S. Pat. No. 4,440,871; J. Chen et al. in “Studies in Surface Science and Catalysis”, Vol. 84, pp. 1731-1738; U.S. Pat. No. 5,279,810; J. Chen et al. in “Journal of Physical Chemistry”, Vol. 98, pp. 10216-10224 (1994); J. Chen et al. in “Catalysis Letters”, Vol. 28, pp. 241-248 (1994); A. M. Prakash et al. in “Journal of the Chemical Society, Faraday Transactions” Vol. 90(15), pp. 2291-2296 (1994); Yan Xu et al. in “Journal of the Chemical Society, Faraday Transactions” Vol. 86(2), pp. 425-429 (1990).
Regular crystalline molecular sieves, such as the CHA framework type materials, are built from structurally invariant building units, called Periodic Building Units, and are periodically ordered in three dimensions. Disordered structures showing periodic ordering in less than three dimensions are, however, also known. One such disordered structure is a disordered planar intergrowth in which the building units from more than one framework type, e.g., both AEI and CHA, are present. One well-known method for characterizing crystalline materials with planar faults is DIFFaX, a computer program based on a mathematical model for calculating intensities from crystals containing planar faults (see M. M. J. Tracey et al., Proceedings of the Royal Chemical Society, London, A [1991], Vol. 433, pp. 499-520).
International Patent Publication No. WO 02/70407, published Sep. 12, 2002 and incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, now designated EMM-2, comprising at least one intergrown form of molecular sieves having AEI and CHA framework types, wherein said intergrown form has an AEI/CHA ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of said silicoaluminophosphate molecular sieve. EMM-2 has been found to exhibit significant activity and selectivity as a catalyst for the production of light olefins from methanol (MTO).
U.S. Pat. No. 6,334,994, incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, referred to as RUW-19, which is also said to be an AEI/CHA mixed phase composition. In particular, RUW-19 is reported as having peaks characteristic of both AEI and CHA structure type molecular sieves, except that the broad feature centered at about 16.9 (2θ) in RUW-19 replaces the pair of reflections centered at about 17.0 (2θ) in AEI materials and RUW-19 does not have the reflections associated with CHA materials centered at 2θ values of 17.8 and 24.8. DIFFaX analysis of the X-ray diffraction pattern of RUW-19 as produced in Examples 1, 2 and 3 of U.S. Pat. No. 6,334,994 indicates that these materials are characterized by single intergrown forms of AEI and CHA structure type molecular sieves with AEI/CHA ratios of about 60/40, 65/35 and 70/30. RUW-19 is reported to be active as a catalyst in the production of light olefins from methanol (MTO).
Study of the synthesis of EMM-2 has shown that the crystallization process to produce such AEI/CHA intergrowths proceeds through the formation of a (silico)aluminophosphate hydrate precursor, such as ALPO-H3 and/or variscite and/or metavariscite, during heat-up of the mixture, followed by dissolution of the precursor as the intergrown molecular sieve nucleates. Unless the temperature and time of nucleation and the amount of directing agent in the synthesis mixture are sufficient, it has been found that significant quantities of the hydrate can be present in the as-synthesized product.
On heating (silico)aluminophosphate hydrates undergo dehydration to produce other crystalline phases, such as phosphocristobolite, phosphotridymite, ALPO-A, ALPO-C, ALPO-D, ALPO-E, dehydrated variscite and dehydrated metavariscite. For example, ALPO-H3 forms ALPO-C on dehydration. ALPO-C has the APC structure having two sets channels defined by eight-membered rings of tetrahedrally coordinated atoms with pore sizes of about 0.34×0.37 nm and about 0.29×0.57 nm. In contrast, both CHA and AEI structure type materials have a pore size of about 0.38×0.38 nm. Thus it was believed that the presence of such impurity phases would be deleterious to the catalytic properties of the intergrowth and so the synthesis process should be closely controlled to avoid the production of these impurities.
Surprisingly, it has now been found that a catalyst composition comprising an AEI/CHA intergrowth exhibits excellent selectivity to ethylene and propylene in oxygenate conversion even if the catalyst composition also contains one or more additional crystalline phases resulting from the dehydration of (silico)aluminophosphate hydrate(s) produced during synthesis of the intergrowth. This finding significantly simplifies the synthesis of AEI/CHA intergrowths by, for example, allowing crystallization temperature and/or time to be reduced and allowing the amount of expensive directing agent required to synthesize the intergrowth to be reduced.
Aluminophosphate hydrates, such as ALPO-H3, and their transformation on heating to other crystalline phases, such as ALPO-C, are disclosed in, for example, F. d'Yvoire, “Etude des phosphates d'aluminium et de fer trivalents. 1. L'orthophosphate neutre d'aluminium”, Bull. Soc. Chim. France (1961) 1762; E. B. Keller et al., “Synthesis, structures of ALPO4-C and ALPO4-D and their topotactic transformation”, Solid State Ionics 43 (1990) 93-102; L. Canesson et al., “Synthesis and characterization of cobalt-containing hydrated aluminophosphate molecular sieves CoAPO4-H3”, Microporous and Mesoporous Materials 25 (1998) 117-131; B. Duncan et al., “Template-free synthesis of the aluminophosphates H1 through H4”, Bull. Soc. Chim. Fr. (1992), 129, 98-110 and K. Kunii et al., “Template-free synthesis and adsorption properties of microporous crstal ALPO4-H3”, Microporous and Mesoporous Materials 50 (2001) 181-185.
U.S. Pat. No. 6,531,639 discloses a method of making an olefin product from an oxygenate-containing feedstock by contacting the feedstock with a non-zeolite catalyst at an oxygenate partial pressure of greater than 20 psia, a weight hourly space velocity of greater than 2 hr−1, an average gas superficial velocity of greater than 1 meter per second, and an oxygenate proportion index of at least 0.5. The catalyst employed is a silicoaluminophosphate (SAPO) molecular sieve selected from SAPO-5, SAPO-8, SAPO-11, SAPO-16, SAPO-17, SAPO-18, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-36, SAPO-37, SAPO-40, SAPO-41, SAPO-42, SAPO-44, SAPO-47, SAPO-56, metal-containing forms, mixtures and intergrowths thereof. In addition, further olefin-forming molecular sieve materials can be included as a part of the SAPO catalyst composition or as separate molecular sieve catalysts in admixture with the SAPO catalyst if desired. Examples of suitable small pore molecular sieves are said to include AEI, AFT, APC, ATN, ATT, ATV, AWW, BIK, CAS, CHA, CHI, DAC, DDR, EDI, ERI, GOO, KFI, LEV, LOV, LTA, MON, PAU, PHI, RHO, ROG, and THO structure type materials, whereas examples of suitable medium pore molecular sieves are said to include MFI, MEL, MTW, EUO, MTT, HEU, FER, AFO, AEL and TON structure type materials.